1. Field of the Invention
The present invention relates to an actuator and its control method, and a lens device, and particularly relates to an actuator which is loaded on a compact precision apparatus such as a digital camera and a cellular phone to drive a zoom lens and a focus lens.
2. Related Art
There is an actuator using a piezoelectric element as a drive device of a lens part of a digital camera or the like. For example, in an actuator in Japanese Patent No. 2633066, a driving rod is fixed to an end surface of the piezoelectric element and a lens-barrel is slidably supported at the driving rod. A plate spring is mounted to the lens-barrel so that the frictional force works between the lens-barrel and the driving rod by the elastic force of the plate spring. A driving pulse in a substantially sawtooth-shaped waveform is applied to the piezoelectric element, and the piezoelectric element deforms at different speeds in an extending direction and a contracting direction. When, for example, the piezoelectric element gradually deforms, the lens-barrel moves with the driving rod. On the other hand, when the piezoelectric element deforms fast, the lens-barrel stays at the same position by the inertia of its mass. Accordingly, by repeatedly applying the driving pulse in the substantially sawtooth-shaped waveform to the piezoelectric element, the lens-barrel can be intermittently moved at a fine pitch.
However, the actuator described in Japanese Patent No. 2633066 transmits a driving force via the long driving rod, and therefore, vibration of the piezoelectric element is absorbed by the driving rod and attenuated, thus causing the problem of being unable to move the lens-barrel accurately. The vibration at a high frequency especially has a large attenuation rate by the driving rod, and therefore, responsiveness of the lens-barrel becomes unfavorable. Therefore, the actuator in Japanese Patent No. 2633066 can be controlled only with the driving pulse at a low frequency, thus causing the problem of the number of moving times of the lens-barrel per unit time becoming small. Therefore, in order to increase the moving speed of the lens-barrel in the actuator in Japanese Patent No. 2633066, it is necessary to increase the applied voltage to increase the displacement amount of the piezoelectric element, and increase the moving amount of the lens-barrel at one time.
In Japanese Patent Application Laid-open No. 2000-50660, the moving amount at one time is increased by raising the power supply voltage of 5 V to 30 V, and thereby the moving speed of the lens-barrel is increased. Therefore, in Japanese Patent Application Laid-open No. 2000-50660, a boosting device becomes necessary, thus causing the problems of the device becoming large in size, and requiring a complicated control.
In an actuator described in Japanese Patent Application Laid-open No. 10-232337, an engaging member is mounted to an end surface in the displacing direction of the piezoelectric element, and this engaging member is frictionally engaged with the movable plate, and the lens-barrel is mounted to the movable plate. By applying the driving pulse to the piezoelectric element, the vibration is transmitted via the engaging member, and the movable plate and the lens-barrel move.
Incidentally, in the actuators disclosed in Japanese Patent No. 2633066, Japanese Patent Application Laid-open No. 2000-50660, and Japanese Patent Application Laid-open No. 10-232337, the speed difference between extension time and the contraction time has to be set so that the magnitude relation of the frictional force of the driving member (the above-described driving member, engaging member and the like) and the driven member (the above-described lens-barrel, moving plate and the like) and the inertia force of the driven member is inverted at the extension time and the contraction time of the piezoelectric element. Accordingly, there is the problem that selection of the spring force which frictionally engages the driven member and the driving member with a suitable frictional force is extremely difficult. Especially in Japanese Patent Application Laid-open No. 10-232337, the spring force is generated by the shape of the driving member (engaging member), and therefore, it is very difficult to set the suitable spring force. Therefore, Japanese Patent Application Laid-open No. 10-232337 has the possibility that the driven member does not slide because the frictional force becomes large, and the driven part does not move because the frictional force becomes small, as a result of which, the driven member cannot be moved accurately.
In the actuator disclosed in Japanese Patent Application Laid-open No. 10-232337, the movable plate is held from both sides with the engaging member to be frictionally engaged, and there is the problem that a rattle occurs to the movable plate in the direction orthogonal to the holding direction. Therefore, there arises the problem that the movable plate (driven member) cannot be accurately moved.
Further, the actuators disclosed in Japanese Patent No. 2633066, Japanese Patent Application Laid-open No. 2000-50660, and Japanese Patent Application Laid-open No. 10-232337 have the problem that the frictional forces between the driving member (the above described driving rod, engaging member and the like) and the driven member (the above-described lens-barrel, moving plate and the like) easily become unstable. Especially when machining accuracy of the driving member and the driven member is low, there is the possibility that the aforesaid frictional force changes in accordance with the contact position of the driven member and the driving member, and the frictional force changes in accordance with the individual actuators. Therefore, the conventional actuators have the problem that it is difficult to move the driven member stably and accurately.
When the actuators disclosed in Japanese Patent No. 2633066, Japanese Patent Application Laid-open No. 2000-50660, and Japanese Patent Application Laid-open No. 10-232337 are applied to lens devices, position detectors which detect the position of the lens-barrel are needed. As the position detector, for example, a transmission type photo-interrupter, and a reflection type photo-reflector are used, and based on the detection values of the position detectors, drive control of the actuator is performed.
The photo-interrupter and the photo-reflector themselves are very compact, but a shielding body and a reflection plate are large, and thus causing the problem that the lens device becomes large in size.